Annals of Botany 2008 102(3):vii; doi:10.1093/aob/mcn148
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John Bryant takes a closer look at some of this month's Original Articles
J. A. Bryant, Professor
University of Exeter, UK
E-mail j.a.bryant{at}exeter.ac.uk
Gliadin gene control – comfort comes closer for celiac sufferers
There is little
doubt that the incidence of diagnosis of celiac (cœliac)
disease – mainly caused by allergy to wheat
-gliadins
– is increasing. Whether this increase arises, as some
suggest, from the way that bread is manufactured on a large
scale or whether detection of the condition has improved is
not clear.
Van Herpen et al. (a joint UK–Netherlands research team; pp. 331–342) cite previous results showing that the allergic reactions exhibited
by sufferers are elicited by particular epitopes in the
-gliadins
encoded in the D-genome and, to a lesser extent, in the A-genome
of wheat. Gliadins encoded by the B-genome are much less allergenic.
Further, there is evidence that the timings of deposition during
grain development of the
-gliadins encoded by the three genomes
are subtly different. With this knowledge comes the possibility
that the allergenicity of wheat
-gliadins could be much reduced
by altering the proportions of the total
-gliadin content encoded
by the three genomes. The authors used a 592-bp fragment of
the promoter of an
-gliadin gene in wheat's B-genome to drive
the synthesis of a marker protein, β-glucuronidase (GUS)
in transgenic plants. As expected, GUS was deposited in the
starchy endosperm and in the sub-aleurone but, unexpectedly,
also in the aleurone itself. Since
-gliadins were not detected
in the aleurone this suggests that other controls operate in
addition to those based on this promoter fragment, possibly
mediated by sequences further upstream. There are also sequence
differences between
-gliadin promoters from the A- and B-genomes.
Unfortunately the databases do not yet contain sequences from
any D-genome
-gliadin promoters. Nevertheless, the differences
in temporal patterns of deposition, coupled with the already
known differences in promoter sequences raise the possibility
of using GM techniques to modify the proportions of the different
-gliadins to the benefit of celiac disease sufferers. The hope
is that this can be done without affecting adversely the physico-chemical
properties of wheat flour.
Sugar maple succumbs to strength-sapping stress
On my first visit
to the USA many years ago, I was amazed to see that many of
the trees in the areas of Virginia close to Washington DC were
harbouring structures that looked like small, woven tents. A
local resident informed me that, indeed, these were the homes
of ‘tent caterpillars’ and that the infestation
was particularly bad. I have seen these tents on some of my
subsequent trips but certainly not at the frequency I observed
on that first visit. Questions obviously arise about the amount
of damage done by these herbivores. Are there any long-term
effects on the life of the tree? Based on the careful and thorough
analysis carried out by
Hartmann and Messier (Montréal; pp. 377–387) the answer to the latter question is clearly ‘yes’.
They have compared past growth rates (as indicated by growth-ring
analysis) in living and recently (post-1993) dead sugar maple
(
Acer saccharum) trees. Growth rates were related to known previous
infestations by forest tent caterpillars (larvae of
Malacosoma distria) and to soil/root disturbance caused by a thinning out
(‘partial harvest’) that affected some trees 10–11
years prior to the growth-ring measurements. It was very clear
that an infestation by tent caterpillars strongly reduced both
growth and vigour. A further infestation, even if it occurred
more than 10 years later, led to a further decline in vigour.
Trees most badly affected by previous infestations were very
likely to die if they suffered another defoliation. Furthermore,
this decline was accelerated in those trees that had been disturbed
by the partial harvest. Interestingly, the partial harvest itself,
in the absence of tent caterpillar infestation, did not cause
any significant decline in growth or vigour. Overall, the authors
conclude that the data are consistent with Manion's tree disease
model [Manion P. 1981.
Tree disease concepts. Englewood Cliffs,
NJ: Prentice Hall] where decline and death are ‘driven
by an interaction between predisposing and inciting stresses’.
Can CAM cope with copious carbon dioxide?
One of the main
causative agents of global warming, namely the increasing atmospheric
CO
2 concentration, will have an effect on plant growth aside
from any other effects of climate change. In general, it is
expected that higher CO
2 concentrations will lead to increased
photosynthetic carbon fixation in C
3 plants, at least partly
because photorespiration will be suppressed. C
4 plants, with
their PEP carboxylase-based CO
2 concentrating mechanism, are
much less likely to benefit. But what about CAM plants? Although
much of their CO
2 fixation occurs at night via PEP carboxylase,
significant C
3 photosynthetic activity may occur in the light
period and it is possible that day-time CO
2 fixation will be
stimulated. To investigate this and other aspects of CAM,
Ceusters et al. (Katholieke Universitiet, Leuven, Belgium and Newcastle University, UK; pp. 389–397) have grown a CAM bromeliad (an
Aechmea hybrid) in 700 µmol
mol
–1 CO
2 (concentration expected by the middle of the
century) for 5 months. This led to a 60 % increase in carbon
gain over each 24-h period. The main effects were in phase II
(early in the light period) and especially in phase IV. Proportionally,
the greater increases by far were in C
3 fixation, although,
perhaps unexpectedly, day-time C
4 fixation was also stimulated.
There was no stimulation of night-time C
4 fixation. Further,
because stomatal conductance was lower under elevated CO
2, water
use efficiency was two-fold higher over the full 24-h period.
Intriguingly, none of the extra fixed carbon was exported to
produce increased biomass. Indeed, day-time export of sugars
was abolished, although this was balanced by an increase in
night-time export. It seems that much of the extra carbon was
kept as hexose in order to provide substrates for the CAM cycle.
The authors thus suggest that ‘whilst some CAM species
[such as
Aechmea] may not show enhanced biomass production in
a higher CO
2 world, productivity could be maintained with reduced
inputs of water’.
Light and leaves – periwinkle proves DAT's the way to do it
It is estimated
that even in modern western medicine, 25 % of prescribed drugs
are derived from plants. Admittedly over half of these are now
manufactured synthetically but that still leaves a large number
that are extracted from plants. Although many of these pharmaceuticals
may be produced in cell cultures, others are only synthesized
in specific types of cell. One example is the monoterpene-indole
alkaloid (MIA), vindoline in
Catharanthus roseus, as discussed
by
Campos-Tamayo et al. (Yucatan, Mexico; pp. 409–415).
Catharanthus roseus produces over 100 MIAs but only vindoline
requires both light and the specialized cell organization of
the aerial parts of the plant. Both these regulatory factors
act via the activity of the last enzyme in the pathway, deacetylvindoline
acetyl CoA acetyltransferase (DAT). To study the effects of
both light and morphogenesis, the authors exposed shoot cultures
to continuous light or to a 16-h photoperiod, monitoring the
formation of new plantlets and the synthesis of vindoline over
a 36-d period. Here we focus on the photoperiod experiments.
Plantlet formation occurred in waves and increases in vindoline
accumulation were correlated with these waves, as were peaks
of DAT activity. Enzymes acting earlier in the MIA pathway did
not show this correlation. However, peaks of DAT activity were
not associated with peaks in transcription of the
dat gene.
This suggests that the changes in enzyme activity were at least
in part mediated post-transcriptionally. Further, in these shoot
cultures, vindoline synthesis is independent of the ORCA3 transcription
factor, involved in the induction of MIAs by jasmonate. Finally,
the authors used plant hormones to disrupt the genesis of new
plantlets, leading to the formation of ‘de-differentiated’
cultures. Vindoline accumulation ceased and the amount in the
cultures fell to zero within 14 d, as did the activity of DAT.
These data thus emphasize the linkage between morphogenesis
and the synthesis of a specific secondary metabolite.
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